1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, performing various control modes such as an anti-skid control, a traction control and a steering control for restraining an excessive oversteer and excessive understeer which will occur during, for example, cornering, by applying a braking force to each wheel of the vehicle irrespective of depression of a brake pedal.
2. Description of the Related Arts
In a recent vehicle, a hydraulic braking system is designed for performing not only a normal braking but also an anti-skid control and a traction control. For example, Japanese Patent Laid-open Publication No. 5-185918 discloses a hydraulic braking apparatus to perform the skid control and traction control. Referring to figures in the Publication, the apparatus includes a first wheel cylinder (18) which is mounted on a driven wheel to apply a braking force thereto, a second wheel cylinder (20) which is mounted on a non-driven wheel to apply the braking force, and a master cylinder (15) for generating a pressurized brake fluid. The master cylinder (15) is connected to the first wheel cylinder (18) through a brake passage (17), in which a normally open inlet valve (34) is disposed. The first wheel cylinder (18) is connected to an accumulator (43) through a normally closed outlet valve (40). A fluid pump (41) has an inlet which is connected to the accumulator (43), and an outlet which is connected to the brake passage (17) between the master cylinder (15) and the inlet valve (34). The fluid pump (41) is provided for pressurizing the brake fluid in the accumulator (43), and discharging it into the first wheel cylinder (18).
In the brake passage (17) between the outlet of the pump (41) connected thereto and the master cylinder (15), is disposed an orifice (30), through which the pressurized brake fluid is gradually supplied to the first wheel cylinder (18) during the anti-skid control operation. In the brake passage (17) between the orifice (30) and the master cylinder (15), is disposed a normally open first cut-off valve (27) which is closed during the traction control operation. The brake passage (17) between the valve (27) and the master cylinder (15) is connected to the inlet of the pump (41) through an induction passage (44), in which a normally open second cut-off valve (50) is disposed to be closed during the anti-skid control operation, and to be opened during the traction control operation. Therefore, the brake fluid in the master cylinder (15) is introduced into the pump (41) through the induction passage (44) to be pressurized and discharged into the first wheel cylinder (18) through the brake passage (17) without the orifice. The induction passage (44) between the valve (50) and the inlet of the pump (41) is connected to the first wheel cylinder (18) through an additional depression brake passage (51), whereby the additional depression of the bake pedal can be made by a vehicle driver during the traction control operation.
According to the above-described apparatus, the valve (50) is generally opened during the traction control operation, but it has to be closed when the brake fluid stored in the accumulator (43) is drained by the pump (41). In the latter case, however, the brake fluid in the wheel cylinders (18) can not be pressurized, even if the brake pedal is depressed.
Furthermore, according to the above-described apparatus, the second wheel cylinder (20) is not connected to the outlet of the pump (41), because the traction control is not performed with respect to the non-driven wheel, on which the second wheel cylinder (20) is mounted.
In addition to the anti-skid control and traction control as describe above, the recent vehicle is provided with a control system for controlling the braking force applied to the wheel irrespective of depression of the brake pedal (i.e., even when the vehicle driver does not operate the brake pedal) to perform a front-rear braking force distribution control, a steering control by braking as described later in detail, and an auto-braking control. In the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle. The apparatus is adapted to control the braking force applied to the vehicle by the braking force control system in response to a comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering.
In general, "oversteer" and "understeer" are used to indicate a vehicle steering characteristic. When the oversteer is excessive during vehicle motion such as cornering, the rear wheels of the vehicle have a tendency to slip excessively in the lateral direction to cause a decrease in turning radius of the vehicle. The oversteer occurs when a cornering force CFf of the front wheels largely exceeds a cornering force CFr of the rear wheels (i.e., CFf&gt;&gt;CFr). As shown in FIG. 17, when a vehicle VL is undergoing a cornering maneuver along a curve of a turning radius R, for example, a lateral acceleration Gy which is normal to the vehicle's path of motion is calculated in accordance with an equation of Gy=V.sup.2 /R, where "V" corresponds to a vehicle speed, and a total CFo of the cornering force is calculated in accordance with the following equation: EQU CFo=.SIGMA.CF=m.multidot.Gy
where "m" corresponds to a mass of the vehicle VL. Accordingly, in the case where the sum of the cornering force CFf and the cornering force CFr is larger than the total cornering force CFo for the vehicle's cornering maneuver along the curve of the turning radius R (i.e., CFo&lt;CFf+CFr), and the cornering force CFf of the front wheels largely exceeds the cornering force CFr of the rear wheels (i.e., CFf&gt;&gt;CFr), i.e., the oversteer is excessive, then the vehicle VL will be forced to turn in a direction toward the inside of the curve in the vehicle's path to cause a reduce in turning radius of the vehicle VL as shown in FIG. 17.
When the understeer is excessive during cornering, the lateral slip of the vehicle will be increased, the vehicle VL will be forced to turn in a direction toward the outside of the curve in the vehicle's path to cause an increase in turning radius of the vehicle VL as shown in FIG. 18. Thus, the excessive understeer occurs when the cornering force CFf of the front wheels is almost equal to the cornering force CFr of the rear wheels so as to be balanced with each other, or the latter is slightly larger than the former (i.e., CFf&lt;CFr), and when the sum of the cornering force CFf and the cornering force CFr is smaller than the total cornering force CFo which is required for the vehicle's cornering maneuver along the curve of turning radius R (i.e., CFo&gt;CFf+CFr), then the vehicle VL will be forced to turn in the direction toward the outside of the curve in the vehicle's path thereby to increase the turning radius R.
The excessive oversteer is determined on the basis of a vehicle side slip angle or vehicle slip angle .beta. and a vehicle slip angular velocity D.beta., for example. When it is determined that the excessive oversteer occurs during cornering, a braking force will be applied to a front wheel located on the outside of the curve in the vehicle's path, for example, to produce a moment for forcing the vehicle to turn in the direction toward the outside of the curve, i.e., an outwardly oriented moment, in accordance with an oversteer restraining control, which may be called as a vehicle stability control. On the other hand, the excessive understeer is determined on the basis of a difference between a desired lateral acceleration and an actual lateral acceleration, or a difference between a desired yaw rate and an actual yaw rate, for example. When it is determined that the excessive understeer occurs while a rear-drive vehicle is undergoing a cornering maneuver, for example, the braking force will be applied to a front wheel located on the outside of the curve and applied to both of the rear wheels to produce a moment for forcing the vehicle to turn in the direction toward the inside of the curve, i.e., an inwardly oriented moment, in accordance with an understeer restraining control, which may be called as a course trace performance control. The above-described oversteer restraining control and understeer restraining control as a whole may be called as a steering control by braking. In accordance with a similar manner as described heretofore, an auto-braking control may be performed to apply the braking force to the vehicle wheels automatically when a frontal obstacle is detected.
In the vehicle motion control system as described above, the hydraulic pressure in the wheel cylinders is controlled by means of the auxiliary pressure source, with the hydraulic communication between the master cylinder and the wheel cylinders blocked. For example, when the steering control by braking is performed, the hydraulic pressure in the wheel cylinders is controlled irrespective of the condition of the brake pedal. During the steering control by braking is being performed, however, if the brake pedal is depressed, it is desirable to immediately shift a control mode to a normal braking mode with respect to at least a wheel which is not being controlled. In order to detect the depression of the brake pedal, it is proposed to use a pressure sensor for detecting the hydraulic pressure. However, the pressure sensor is relatively expensive comparing with the electromagnetic valve, so that it is desirable to reduce the number of the pressure sensors as small as possible.
According to a hydraulic braking system having a master cylinder with a vacuum booster and an auxiliary pressure source (e.g., a fluid pump driven by an electric motor), for example, during the steering control by braking is being performed, if the rapid braking operation is made in response to depression of the brake pedal before the auxiliary pressure source is activated to provide sufficient pressure for a desired braking force distribution, it will be impossible to apply the desired braking force to the wheel to be controlled.
Furthermore, according to the apparatus as disclosed in the Japanese Publication No. 5-185918, the second wheel cylinder (20) mounted on the non-driven wheel is not connected to the outlet of the pump (41), so that the braking force can not be applied to the non-driven wheel without the brake pedal depressed, while it is necessary to apply the braking force irrespective of depression of the brake pedal for performing the steering control by braking and the auto-braking control as described above.